CAS systems are well known. CAS systems use compressed air to drive a turbine, which in turn powers an electrical generator. Prior to reaching the turbine, the compressed air may be heated using a suitable type of fuel-combustion system. Alternatively, an exhaustless heater may be used to heat the compressed air. This type of CAS, which uses an exhaustless heater, is known as a combined thermal and compressed air storage (TACAS) system. Such systems are disclosed in a commonly-assigned, co-pending U.S. patent application Ser. No. 10/361,729, filed Feb. 5, 2003, entitled “Thermal and Compressed Air Storage System,” which is hereby incorporated by reference in its entirety.
When the turbine is driven by the compressed air, heated or not, the turbine powers an electrical generator that produces electrical power at an output. The use of CAS systems alone to provide backup power, however, is not practical in applications where, for example, even a very brief power outage to a load is detrimental. In CAS systems, there typically is a slight delay because the rotor of the turbine must be sped up before the turbine is able to power the electrical generator. This renders the use of CAS systems, by themselves, an unacceptable manner in which to provide backup power in many applications.
Energy storage systems, on the other hand, provide substantially instantaneous backup power to a load in the event of a primary power source failure. An example of an energy storage system is a bank of chemical batteries, which includes one or more chemical batteries. In order to maintain backup power capability, these batteries are either replaced once drained or charged during normal operating conditions (e.g., when utility power is providing sufficient power). In the latter case, the bank of chemical batteries is connected to a battery charger which provides a trickle charge to keep the batteries energized during normal operating conditions. The energy stored in the chemical batteries is then used to supply power to the load during a utility power failure.
Chemical batteries, however, suffer from various deficiencies, including bulkiness, lack of reliability, limited lifespan, temperature sensitivity, high maintenance costs and relatively low safety. For example, chemical batteries require relatively constant and complex recharging, depending on the type of batteries involved, to insure that the batteries continue to operate efficiently and maintain their full storage capacity. Moreover, chemical battery banks must typically be located in remote battery storage rooms which house the batteries, in part due to safety considerations and bulkiness, and must be replaced approximately every 3-8 years due to the limited lifespan of the batteries. Additionally, high maintenance costs arise from the need to install special venting and air-conditioning systems for dedicated battery storage rooms.
Another commonly used type of energy storage system is a flywheel energy storage system. During normal operating conditions, a flywheel is rotated by the primary power source such that it stores kinetic energy in the form of rotational momentum (see, e.g., Clifton et al. U.S. Pat. No. 5,731,645, which is hereby incorporated by reference herein in its entirety). When the primary power fails, the kinetic energy stored in the flywheel is used to drive a generator, which provides the load with backup power. Flywheel energy storage systems, however, are only capable of supplying backup power to the load for a relatively short period of time (e.g., until the kinetic energy in the flywheel has been used up). Once the energy stored in the flywheel energy storage system is depleted, backup power is no longer available for the critical load.
Energy storage systems such as described above are often used in uninterruptible power supply (UPS) systems, which are used to ensure that an interruption in power from the primary power source (e.g., a utility power failure) does not lead to disturbance of the power being supplied to the critical load. UPS systems using flywheel energy storage systems, for example, are described in Gottfried U.S. Pat. No. 4,460,834. Alternatively, Pinkerton et al. U.S. Pat. No. 6,255,743 describes a UPS system which utilizes a turbine energy storage system, while Pinkerton et al. U.S. Pat. No. 6,192,687 describes a UPS system which utilizes a source of thermal energy to produce backup electrical power.
Generally, when a critical load is being powered by utility power, known UPS systems store energy in an energy storage system, or bridging energy system. Thereafter, during a failure in utility power (i.e., when the utility power source is not able to provide power at a predetermined quantity or quality level), these UPS systems begin supplying backup power to the critical load using the energy stored in the energy storage system. Moreover, persons skilled in the art will appreciate that power conditioning or other typical UPS features may be included to further enhance the ability to provide continuous power to the critical load.
The UPS systems described above, however, suffer from various deficiencies. Known flywheel-UPS systems, for example, have only a limited supply of backup energy. UPS systems using battery banks, moreover, are problematic because they suffer from over-temperature conditions when utility power is not present to power heating, ventilation, and air conditioning (HVAC) systems.
In view of the foregoing, it is an object of this invention to provide backup energy systems which provide undisturbed power to a critical load while eliminating problems associated with known backup energy systems.